1. Field of the Invention
The invention relates to ethylene-vinyl acetate copolymer (hereinafter referred to as EVAc) hot-melt adhesives and more particularly relates to such adhesives having extended molten stability and to methods of their preparation.
2. Brief Description of the Prior Art
EVAc-based hot-melt adhesives, their preparation and methods of use are well known in the art; see for example U.S. Pat. No. 3,560,420. These adhesives compositions are used extensively in packaging, laminating and bookbinding. However, their use in typical application apparatus is impaired by the tendency of these hot-melt adhesive compositions to exhibit relatively short pot-lives; i.e.; fluid stability under molten conditions. Generally viscosity increases or gelation and a "skinning" over of air exposed surfaces occurs within relatively short periods of time after initial melting. Almost all EVAc adhesives contain a thermoplastic, modifying resin as a major component which is known as a tackifier. Tackifiers increase the adhesion and lower the viscosity of hot-melt adhesives. The improvement in adhesion is believed to be a result of improved wetting.
Hot melt EVAc adhesives tackified with hydrocarbon resins as the modifying resin are subject to degradation due to oxidation while being maintained in the molten state. Such oxidation frequently causes a dispersion of insoluble gel particles to develop with or without a large change in adhesive viscosity. The dispersed particles clog filter screens and ultimately proceed to a condition where the adhesive gels, clogs the applicator equipment or loses much of its expected strength properties. It is necessary to stabilize these adhesives against oxidation by incorporating into them an oxidation inhibitor such as butylated hydroxy toluene (BHT). A given inhibitor alone may not stabilize an EVAc hot melt adhesive tackified with hydrocarbon resin. It may be necessary to increase the level of antioxidant initially selected, to a higher level as testing reveals that the stability obtained is inadequate. Or it may be necessary to add a different type of antioxidant. A vast number of antioxidants are available and their effects are often synergistic. This subject is well reviewed under the heading "Antioxidants and Antiozonants" by P. O. Nicholas, et al in Volume 3, Third Edition of the Kirk-Othmer Encyclopedia of Chemical Technology.
I have discovered that the presence of soluble lithium ions in hydrocarbon resin tackified hot-melt EVAc adhesive compositions retards the oxidative degradation process. This effect is unexpected because lithium compounds are not generally recognized as antioxidants and in fact they do not retard oxidation, as described above, to the maximum degree in the absence of a conventional anti-oxidant.
The method of the invention is advantageous in that it significantly increases the hot melt stability of EVAc-based hot-melt adhesives containing hydrocarbon resins as a tackifier. This permits an operator to employ apparatus with a greater time latitude and to melt the adhesive compositions well in advance of its application. This provides more flexibility in use of the adhesives.